THE DIRECTION OF MOTION. 249 



among aggregations of men, the like will hold good. The 

 changes in a society, being due to the joint actions of its 

 members, the courses of such changes will be determined 

 as are those of all other changes wrought by composition 

 of forces. 



Thus when we contemplate a society as an organism, and 

 observe the direction of its growth, we find this direction to 

 be that in which the average of opposing forces is the least. 

 Its units have energies to be expended in self -maintenance 

 and reproduction. These energies are met by various en- 

 vironing energies that are antagonistic to them those of 

 geological origin, those of climate, of wild animals, of other 

 human races with whom they are at enmity or in competi- 

 tion. And the tracts the society spreads over, are those in 

 which there is the smallest total antagonism. Or, reducing 

 the matter to its ultimate terms, we may say that these social 

 units have jointly and severally to preserve themselves and 

 their offspring from those inorganic and organic forces 

 which are ever tending to destroy them (either indirectly 

 by oxidation and by undue abstraction of heat, or directly 

 by bodily mutilation) ; that these forces are either counter- 

 acted by others which are available in the shape of food, 

 clothing, habitations, and appliances of defence, or are, as 

 far as may be, eluded ; and that population spreads in which- 

 ever directions there is the readiest escape from these forces, 

 or the least exertion in obtaining the materials for resisting 

 them, or both. For these reasons it happens that fer- 



tile valleys where water and vegetal produce abound, are 

 early peopled. Sea-shores, too, supplying a large amount 

 of easily-gathered food, are lines along which mankind 

 have commonly spread. The general fact that, so far as 

 we can judge from the traces left by them, large societies 

 first appeared in those tropical regions where the fruits of 

 the earth are obtainable with comparatively little exertion, 

 and where the cost of maintaining bodily heat is but slight, 

 is a fact of like meaning. And to these instances may be 

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